In digitally televised systems such as video-telephone, teleconference and high definition television systems, a large amount of digital data is needed to define each video frame signal since the video frame signal comprises a sequence of digital data referred to as pixel values. Since, however, the available frequency bandwidth of a conventional transmission channel is limited, in order to transmit the substantial amount of digital data therethrough, it is necessary to compress or reduce the volume of data through the use of various data compression techniques, especially, in the case of such low bit-rate video signal encoders as video telephone and teleconference systems.
One of such techniques for encoding video signals for a low bit-rate encoding system is an object-oriented analysis-synthesis coding technique, wherein an input video image is divided into objects and three sets of parameters for defining the motions, the contours, and the pixel data of each object are processed through different encoding channels.
In processing a contour of an object, contour information is important for the analysis and synthesis of the object shape. A classical coding method for representing the contour information is a chain coding method. The chain coding method, however, requires a substantial amount of bits for the representation thereof, although the method does not incur any loss in the contour information.
To overcome the drawback, there have been proposed several methods to encode the contour information such as a polygonal approximation and a B-spline approximation. One of the disadvantages in the polygonal approximation is the roughness of the representation of the contour. The B-spline approximation is, on the other hand, capable of representing the contour more precisely; however, it requires a high-order polynomial to reduce the approximation error, resulting in an increased overall computational complexity of the video encoder.
One of the techniques introduced to ameliorate such problems associated with the coarse representation of the contour and the increased computational complexity in the above approximation approaches is a contour approximation technique employing a discrete sine transform(DST).
In an apparatus which adopts the contour approximation technique based on the polygonal approximation and the DST, as disclosed in a commonly owned copending application, U.S. Ser. No. 08/423,604, entitled "A CONTOUR APPROXIMATION APPARATUS FOR REPRESENTING A CONTOUR OF AN OBJECT", a number of vertex points are determined and the contour of the object is approximated through the use of a polygonal approximation for fitting the contour by line segments. And, N sample points on each line segment are selected, and an approximation error at each of the N sample points located on each line segment is sequentially calculated in order to obtain a set of approximation errors for each line segment. And each of the approximation errors represents the distance between the contour and the line segment at each of the N sample points. Thereafter, a set of DST coefficients is generated by performing a one-dimensional DST operation on each set of the approximation errors.
Even though it is possible to remedy the rough representation and computational complexity and reduce the volume of transmission data through the use of the DST-based contour approximation, it still remains desirable to further reduce the volume of transmission data in order to efficiently implement a low-bit rate codec system having, e.g., a 64 kb/s transmission channel bandwidth.